The invention relates to a sensor intended to detect the rotational position of a device (e.g. a shaft) which is rotatable preferably by an engine, whereby the position of the rotary shaft of the position detector represents the rotational position of the rotatable device to control the motor which rotates the device, for controlling the motor which rotates the device, and more particularly relates to a cam mechanism between the position detector and the rotatable device.
Previously known in various contexts is the use of a potentiometer as position sensor for detecting the rotational position of a shaft. Potentiometers have the advantage of being easy to manufacture in large series and therefore being inexpensive, at least so long as standardized potentiometers can be used. A disadvantage, however, is that their accuracy is relatively poor. In cases where better accuracy is required, the potentiometer has to be manufactured with greater accuracy at consequently higher cost. One characteristic of potentiometers is that they have the same accuracy along with whole of their relevant rotary movement. In certain cases greater accuracy is only required along a smaller part of the rotary movement and it is possible within other rotational sectors to accept somewhat inferior accuracy of resolution. Although greater accuracy is only required within a limited range, it is necessary in such cases to use a more accurate potentiometer which is then unnecessarily accurate within other ranges.
An example in the vehicle industry where the problem mentioned arises is in motorcars of the convertible type. These motorcars have a folding hatch or cover which is intended to cover the hood in the folded position. This hatch is operated by means of an electric motor which opens and closes the hatch as required. Previously known is the use of a potentiometer for detecting the position of the shaft of the motor which is used for operating the hatch. However, using standard potentiometers results in poor hatch control accuracy. Either the hatch does not close fully or the closing movement ceases too late, with consequent risk of damage to other components. Certain parts are also liable to corresponding problems pertaining to detecting the hatch in its fully open position. The alternative of using a potentiometer with greater accuracy involves the disadvantage of a substantially higher price. The problem is therefore to find a solution whereby the position of the hatch can be detected with sufficient accuracy for being able to stop the drive motor at the exact position particularly of the closed hatch but also to some extent of the open hatch, without having to use an expensive potentiometer. Corresponding problems also arise with other types of position detecting sensors, e.g. inductive sensors.